Source: Molyvann, Vann. Modern Khmer Cities. Phnom Penh, Cambodia : Reyum ; [Chicago, IL?] : Sales and distribution, USA, Art Media Resources, c2003.
“…the first hydraulic city of Angkor, a settlement linking water to economic, social and religious systems…”
“…a city with moats which mirrored, in reduced form, the cosmology of the Brahmanic universe as a series of concentric mountains and oceans.”
“…irrigation networks joined existing forms of irrigation found on the lower plans which made use of the rising flood waters of the Great Lake. The region thus had a double supply of water from the baray (reservoir) to flood its rice fields. With these two sources of water, at least three crops of rice could be produced per year.”
p 46 Description
“B.P. Groslier has described hydraulic cities as based on a highly productive system of irrigation carefully adapted to the region in which these were established… They “baray” filled through the monsoon rains (June to October) and the accompanying flood of the revers; rice planted during the dry season (that is, with sowing and transplanting in September and October, and harvest in January and February) then made full use of the water in the baray. If the monsoon season brought little rain or was too short, the baray along with the Great Lake still allowed the rice fields to be filled with water when necessary. Thus even if the irrigation system tied to the hydraulic city did not always permit two (or even three) harvests, it still could guarantee at least one harvest of rice per season, thus overcoming unforeseen variations in climate.”
p 47 Decline
Hypothesis 1: Geological movement of the tectonic plate of the basin of the Great Lake which could, in the end, have disturbed the water supply of the system.
“SPAFA Journal #- Dr. Heng L. Thung : “Angkor may have been condemned, even before it was constructed, by a slow geological movement which led to a change in the slope of rivers and their beds, rendering the huge Khmer water reservoirs unusable. Over the centuries, the slow, level and meandering rivers which fed these reservoirs changed their profiles and their course. The city depended on reservoirs for its water supply during the dry season which coincided with the retreat of the flood waters of the Lake.”
“Physical evidence for such movements can be found in the landscape of the present. The Siem Reap River was constructed artificially to provide water to the moats of Angkor Wat…The intakes of the baray were originally two meters below the surface, thus matching the original water level of the man-made Siem Reap River….The sinking of the stream bed, accompanied by lower water levels during the dry season, has rendered the intakes to the reservoirs useless as the water level diminished.”
Hypothesis 2: Vices of structure which were inherent in the hydraulic system used at Angkor from the moment it was established, and which in the end caused the general blockage of the system.
A. Deforestation: In methodically extending their rice fields toward Phnom Kulen, the Khmers necessarily deforested large areas of land. The effects of this deforestation, especially on the hills and higher elevations which served as water reservoirs, are well known. Initial rivulets washed away portions of the soil while the remaining soil became compacted. Gullies then formed and large scale erosion of the soil followed. Large areas of bare earth led to a decrease in convection rains once caused by water evaporation from areas of foliage. With less rain, the level of water table decreased.
B. Siltation: The hydraulic system of Angkor depended on gravity. In a flat country where rivers run without a strong current are themselves fed by slow and turbid streams, the waterways inevitably silted up. In the moats, the rate of siltation seems to have been 2-3 millimeters per year. The Wester Baray silted at a rate of .3 meters per century.
C. Siltation of the Lake: Ongoing natural sedimentation in the Lake necessarily, if insidiously, modified the mouths of the rivers. Today the Siem Reap River slows at least two meters lower than the intended entrances of all known Angkorian waterworks due to erosion. This indicates the degree to which siltation has changed the relation of the lake to the rivers flowing into it.
D. Decrease in the Water Table: Linked to the changes described above are modification in the ground water level which has dropped 0.7 meters. As long as irrigation was assured and the water level remained sufficiently high and constant, mineral exchanges through the soil were minimal and clay remained below the surface. When the rivers was dry during the hot season, the ground water level decreased and evaporation brought particles of clay to the surface. Little by little, the arable layer of earth filled with iron and thus was rendered fallow and unusable.
E. Malaria: The stagnant waters of the large reservoirs general became excellent breeding grounds for mosquitos which then devastated local populations with malaria.